1. Field of the Invention
This invention relates to serial printers and more particularly to improvements in a detent apparatus for serial printers having a print wheel with a plurality of type-bearing spokes.
2. Description of the Prior Art
In a serial printer having a disc-shaped print wheel, which has numerous type-bearing spokes located radially and provided with type elements on the tips of the spokes and which is rotated by a motor, a selected spoke having come to a printing position or point is struck behind the type element by a print hammer toward a platen to perform printing.
Printers, in which a positioning mechanism, consisting of a positioning projection and a positioning recess which cooperate with each other during the striking action of a print hammer, is provided between hammer and the spoke or between the spoke and a guide member for striking a selected type element against a printing point on a platen to obtain an orderly row of printed characters, have already been proposed. A printer disclosed in U.S. Pat. No. 3,442,365 exemplifies a printer having a positioning mechanism between the hammer and each spoke. Another printer disclosed in U.S. Pat. No. 3,983,985 exemplifies a printer having a positioning mechanism between each spoke and the guide member.
In such conventional printers, the print wheel is driven by a DC motor or an AC motor which is continuously rotating at a constant speed and the spoke is struck by the print hammer at the moment of the passing of the spoke by the printing position, or the print wheel is driven by a stepping motor having a number of steady-state positions corresponding to the total number of spokes and the spoke is struck by the print hammer while the stepping motor is energized to hold a type element opposite the printing position. However, one deficiency is that the positioning means might not always operate properly or the struck spoke might receive not only a force toward the platen, but also a turning force due to the rotary force or stopping force of the motor during the striking action of the print hammer. As a result, an orderly row of printed characters is not always obtained, or the damage to each spoke resulting from material fatigue, especially its butt portion, is accelerated.
To eliminate these deficiencies, each type element on the print wheel needs to be accurately positioned opposite the printing position during every printing action or operation. For this purpose, it is required that the print wheel be accurately shaped and a detector for detecting the rotational position of the print wheel have very high performance. When a stepping motor, appropriate to step-by-step action, is rotated in two directions to select a type element more quickly, the motor needs to be manufactured so as to minimize the difference between the steady-state positions in both directions of rotation of the motor. These criteria result in complicating the manufacturing and assembly work of serial printers and increasing their cost.